Chromates

Description

Chromates

The starting-point for the chromates is chrome iron ore, Fe(CrO2)2, a spinel (see p. 100). It is heated in a powdered state with a mixture of lime and potassium carbonate, in a reverberatory furnace, where the atmosphere is a strongly oxidising one. The product is a mixture of calcium and potassium chromates and ferric oxide : 2Fe(CrO2)2 + 4K2CO3 + 70 = Fe2O3 + 4K2CrO4 + 4CO2. The fritted mass is treated with water, when the chromate dissolves, leaving the ferric oxide insoluble. On evaporation, potassium chromate crystallises out. If it is desired to produce " bichromate " or anhydrochro-mate of potassium, K2Cr2O7, the solution of the chromate is treated with dilute sulphuric acid ; calcium sulphate is precipitated, and is removed by settling ; on evaporation, sparingly soluble sulphate of potassium crystallises out; and after removal of the crystals, on further evaporation," bi-chrome " crystallises. The conversion of the chromate into the anhydrochromate is represented by the equation: 2K2CrO4.Aq + H2SO4.Aq = K2Cr2O7.Aq +K2SO4.Aq. This conversion is accompanied by a colour-change; for the ions of chromate, =CrO4, are yellow, whereas those of anhydrochromate, =Cr2O7, are orange. On addition of potassium hydroxide to the bichromate, the opposite change takes place; the anhydrochromate ion is changed into the chromate ion :-